A semiconductor laser is utilized in an optical disk apparatus for high-density information recording. A pickup optical system using the semiconductor laser as shown in FIG. 1 may comprise a laser diode 10, a collimator lens 20 for collimating a light beam emitted from the laser diode 10 into a parallel beam, a shaping means 20' and 20" for shaping a beam collimated by the collimator lens 20 into a cross-sectionally circular beam, a beam splitter 30 for splitting the shaped beam, a reflector 40 for deflecting the direction of the split part of the beam, an object lens 50 for forming an image on a disk using the deflected beam, a focusing lens 70 for controlling a focal length of a part of the beam deflected by the splitter, a translucent polarization beam splitter 80 for half-mirroring the beam passing through the focusing lens 70, and photo detectors 90 and 100 for detecting and converting a light half-mirrored by the translucent polarization beam splitter.
A divergent beam emitted from the laser diode 10 has different divergent angles in the directions normal and parallel to the diode junction so that it takes on an elliptic cross section. To reshape the elliptic cross section to a circular one, in this system, a shaping means 20' and 20" must be provided, which is constituted by a cylindrical lens or a triangular prism.
If a cylindrical lens shaper is adopted, as shown in FIG. 3, a collimator lens 20 is used to collimate a beam from the laser diode into a parallel beam and a pair of cylindrical lenses 20A and 20B are used to shape the collimated beam. The beam collimated by the collimator lens has a parallel and normal diameters D1 and D2, the latter being greater than the former, and thus has an elliptic cross-section. The cylindrical lenses 20A and 20B has a power exclusively in the parallel direction and causes the parallel diameter D1 to take on an equivalent dimension as the normal diameter D2, thus forming a cross-sectional circular beam.
If a triangular prism shaper is used, a pair of prisms 25, 25 enlarge the beam in the parallel direction as shown in FIG. 4 by action of combined refractive indices as described below. ##EQU1## where .theta.1 is the incident angle, the refractive index of the incident medium is n, and the refractive index of atmospheric air is n'=1.
Thus, Sin .theta.1=nSin .theta.2. ##EQU2##
Each of the above described beam shaping techniques increase both a length of the total optical path and a size of the optical head. Also, the cylindrical lenses are difficult to align. Further, the triangular prisms are expensive and voluminous, contributing to an increase in the volume and weight of the disk drive. Finally, time is needed to record and reproduce the information therein.